Fuel control systems, of the type described in U.S. Pat. No. 7,137,242, are used in controlling the supply of fuel to an aircraft engine. Such systems have a hydro-mechanical unit (HMU) which contains a metering valve and which receives a supply of fuel at high pressure (HP) from a gear pump driven from, and thus operated at a speed related to, the main shaft of the associated gas turbine engine. The inlet of the gear pump is supplied from a fuel reservoir by means of a boost pump or lift pump, the pressure at the inlet of the gear pump being defined as low pressure (LP), which in practice may be above atmospheric pressure but substantially below HP. The supply line from the outlet of the gear pump contains a fuel filter and enters the HMU to provide an HP supply to the inlet gallery of the metering valve and thence to a variable metering orifice of the valve. As is conventional in metering valves, a spool of the valve is moved within the housing of the valve to control the degree of opening of the metering orifice of the valve and thus the metering of fuel flow through the valve. A delivery line from the metering valve conducts metered fuel at a reduced pressure PX through a pressure raising and shut-off valve (PRSOV) of the HMU. The PRSOV serves in use, to maintain a minimum fuel pump pressure rise (HP−LP), so as to ensure that internal HMU valves and any fuel-pressure operated auxiliary devices (such as variable stator vane actuators, variable inlet guide vane actuators and bleed valve actuators) arranged to receive fuel under pressure from the fuel control system can operate correctly. An output line from the PRSOV exits the HMU to pass the metered fuel to the engine burner manifold(s).
The PRSOV typically contains a spring-biased piston, the front face of which is acted on by fuel at the pressure PX, and the rear face of which is acted on by fuel at further reduced pressure PZ. The level of PZ relative to LP is determined by a PRSOV orifice potentiometer arrangement,
The pressure differential across the gear pump HP−LP can conveniently be used to operate auxiliary engine devices, such as a variable stator vane (VSV) actuator, a variable inlet guide vane (VIGV) actuator and/or a bleed valve actuator. Each actuator is controlled by its own dedicated servo-valve, the pressure differential available to each servo-valve being HP−LP. This pressure differential is made up of three elements:HP−LP=(HP−PX)+(PX−PZ)+(PZ−LP).
At low flow conditions, this is set to a relatively constant value. More particularly, HP−PX, the pressure drop across the metering valve, is generally kept constant e.g. by using a pressure drop control valve (PDCV) and combining spill valve arrangement. PX−PZ, the pressure drop across the PRSOV piston, can be set by the PRSOV spring load and the area of the piston since at low flow conditions the PRSOV piston is active and not at a maximum stop. PZ−LP, the pressure drop across the PRSOV potentiometer return orifice, also generally has a fixed value since the pressure drop across the potentiometer fixed feed orifice (HP−PZ=HP−PX+PX−PZ) is constant setting a fixed flow through both orifices. Thus with typical values at law burner flow conditions of HP−PX=125 psid (0.86 MPa), PX−PZ=70 psid (0.48 MPa), and PZ−LP=245 psid (1.69 MPa), the pressure differential HP−LP available to operate auxiliary engine devices can be about 440 psid (3.03 MPa).
However, the system described above has a number of drawbacks:                It does not separate the HMU and actuator pressure rise requirements. In order to move the actuators at the required velocities against prevailing loads, a high HMU minimum pressure differential HP−LP is needed. This has implications for hydromechanical loop instability, metering valve fail down rates and heat input to the fuel via spill flow from the combining spill valve.        At start conditions (pump speeds around 6-25%), there is a risk to pump bearing integrity. More particularly, operating at a high pressure rise and low speed reduces bearing film thicknesses and can result in excessive pump wear.        At start conditions, the high HP−LP results in high pump/HMU internal leakages back to LP, thereby reducing the amount of pump flow available to start the engine, which can be an issue at low speeds because pump delivery flow is proportional to speed.        With increasing actuator loads on large modern engines, even if the minimum system pressure rise HP−LP is as high as to 440 psid (3.03 MPa), it may still be necessary to introduce larger actuators and servo-valves and this may not be practically possible.        
U.S. Pat. No. 6,176,076 proposes providing a passive restrictor in a spill return line to raise the HMU minimum system pressure rise between start and idle. However, to ensure sufficient pressure at all conditions, generally a small restrictor is required. As a result there can then be too high an HMU pressure rise at other spill conditions, resulting in undesirably increased heat input to the fuel, In addition, the arrangement of U.S. Pat. No. 6,176,076 can be sensitive to external actuator off-take flows. For example, when such external actuators are moved, the reduction in spill flow, and hence reduction in minimum system pressure rise available to move the actuators, can be significant.
Thus there is a need to overcome or avoid such drawbacks while ensuring that fuel is supplied at adequate pressures.